Scanning electron microscope electron-optical column construction

ABSTRACT

A scanning electron microscope in which the electron-optical column is formed by a plurality of magnetic coils disposed around a removable tube, an electron beam being directed axially through the tube. The tube is suitably sealed at respective ends to the electron gun and specimen-containing portions of the electronoptical column, and thus functions to contain the vacuum thereon. Preferably, the tube comprises a thin, non-magnetic stainless steel tube having a wall thickness of less than 10 mils, to prevent electrostatic charging while minimizing eddy current losses. Alternatively, the tube may comprise a thin metallic layer or inner tube within an insulating, reinforcing outer tube, to impart strength and rigidity to the tube structure.

United States Patent Dao et al.

Scanning SCANNING ELECTRON MICROSCOPE ELECTRON-OPTICAL COLUMNCONSTRUCTION Inventors: James'Dao, Alameda; Nelson C.

Yew, Los Altos, both of Calif.

Related US. Application Data Continuation-impart of Ser. No. 75,899,Sept. 28, 1970, abandoned.

US. Cl 250/311, 250/307, 250/397 Int. Cl H01j 37/26, GOln 23/00 Field ofSearch 250/495 A, 49.5 D

References Cited UNITED STATES PATENTS 2/1941 Marton 250/49.5 X 6/1956Le Poole 250/495 X 3/1959 Van Dorsten.... 250/495 10/1969 Kimura et a1.250/495 1/1972 Sato 250/495 X OTHER PUBLICATIONS Electron Microsc py,Thorton, pages [451 Jan. 22, 1974 An Ultrahigh-Vacuum Surface ElectronMicroscope, Miller et a1., IEEE, 9th Annual Symposium on Electron, lonand Laser Beam Technology, pages 118-122.

Primary ExaminerWilliam F. Lindquist Attorney, Agent, or Firm-+Townsendand Townsend [5 7] ABSTRACT A scanning electron microscope in which theelectronoptical column is formed by a plurality of magnetic coilsdisposed around a removable tube, an electron beam being directedaxially through the tube. The tube is suitably sealed at respective endsto the electron gun and specimen-containing portions of theelectron-optical column, and thus functions to contain the vacuumthereon. Preferably, the tube comprises a thin, non-magnetic stainlesssteel tube having a wall thickness of less than 10 mils, to preventelectrostatic charging while minimizing eddy current losses.Alternatively, the tube may comprise a thin metallic layer or inner tubewithin an insulating, reinforcing outer tube, to impart strength andrigidity to the tube structure.

15 Claims, 8 Drawing Figures SCANNING ELECTRON MICROSCOPEELECTRON-OPTICAL COLUMN CONSTRUCTION This application is acontinuation-in-part of our earlier copending application Ser. No.75,899, filed Sept. 28, 1970, entitled Vacuum Containment in ScanningElectron Microscope, now abandoned.

This invention relates to a scanning electron microscope, and, moreparticularly, to the construction of the electron-optical column of ascanning electron microscope.

In a scanning electron microscope, a beam of electrons emitted from acathode are focused upon, and caused to scan, a specimen. The focusingan deflection of the electron beam is usually accomplished by aplurality of magnetic coils which function as lenses and deflectioncoils, and which are arranged successively along the path of theelectron beam to form an electron-optical column. In order to provideappropriate lens-forming magnetic discontinuities between the polepieces of the various coils, the electron-optical column has heretoforebeen constructed in sections, the various sections being interconnectedand sealed to one another to permit operation in the necessary highvacuum environment.

Such a construction suffers from several important drawbacks anddisadvantages. In particular, the provision of a sectionalelectron-optical column has made it extremely difficult to align andadjust the electron lenses with the high degree of accuracy necessary toachieve precision scanning and high resolution of the electron beam.Furthermore, the numerous seals between sections have created a severeleakage problem. In addition, the prior art electron-optical columnconstruction has often placed the magnetic coils in vacuum, resulting insevere out-gassing and a relatively large volume to be evacuated, thusrequiring a long pump-down time. Finally, bombardment of the columnparts by the electron beam often causes contamination which coats andelectrostatically charges the electronoptical column surfaces. Thisimpairs the operation of the electron microscope, and thus requires thatthe electron-optical column be frequently disassembled and cleaned,further complicating the alignment and sealing of the electron-opticalcolumn, and also creating a possibility of damage to the lens elementsduring disassembly and reassembly.

In order to overcome some of the drawbacks thus described, it has beenproposed that a scanning electron microscope be constructed with avacuum containing tube, about which the magnet coils are disposed.Specifically, Scanning Electron Microscopy by Thornton suggests that ascanning electron microscope might, in principle, be constructed with atube interconnecting the electron gun housing and specimen chamber, thecoils being disposed about the tube. This suggestion, while overcomingthe problems of leakage and outgassing of the electron-optical column,ignores the serious problem of contamination of the interior of thetube. Specifically, Thornton suggests no manner in which the tube may beremovably mounted for cleaning and/or replacement. Moreover, use of atube, as suggested by Thornton, introduces a problem of eddy currentlosses due to the fluctuating magnetic fields of the deflection coils,which impairs the operation of the microscope.

Miller and Pease, in an article entitled An Ultra High-Vacuum SurfaceElectron Microscope, I.E.E.E. 9th Annual Symposium on Electron, Ion andLazar Beam Technology, disclosed a scanning electron microscope having atube removably disposed interior of the electron-optical column. Onceagain, the problem of eddy current losses is created, but not solved.Moreover, the tube of Miller and Pease does not extend through theentire electron-optical column, as the final pole piece of the finallens is within vacuum. In order to permit cleaning and/or replacement ofthe tube, the tube is mounted in a removable manner. However, mountingflanges are employed at both ends of the tube which require manualmanipulation. Thus, in order to remove the tube of Miller and Pease,both the specimen chamber and the electron gun housing must bedisassembled. Accordingly, while the tube of the Miller and Peasemicroscope is removable, such removal is a complex and tediousprocedure.

The scanning electron microscopes employing tubes, thus described,suffer from an additional drawback in that the inner wall of the tubetends to reflect stray electrons, thereby detrimentally increasing thebeam spot size.

According to the present invention, there is provided a scanningelectron microscope in which the magnetic coils forming the electronlenses and deflection coils are disposed around a removable tube, theelectron beam being directed axially through the tube. The tube isdemountably sealed at respective ends to the electron gun andspecimen-containing portions of the electron-optical column, and thusfunctions to contain the vacuum. Thus, the magnetic coils are containedoutside of the vacuum, thereby minimizing out-gassing. Furthermore, thevolume which must be evacuated is reduced, and the number of seals areminimized, thereby reducing the pump-down time and minimizing thepossibility of leakage. Moreover, the magnetic pole pieces need nolonger be machined for vacuum sealing, thereby reducing manufacturingcosts.

According to the present invention, one of the demountable seals at theends of the tube is adapted for insertion and/or removal without manualmanipulation. In this manner, the tube may be removed and reinserted forcleaning and/or replacement by merely disassembling one end of theelectron-optical column.

Preferably, the tube comprises a thin wall nonmagnetic stainless steeltube, having a wall thickness of less than ten mils, in order to preventelectrostatic charging, while minimizing eddy current losses due to thedynamic fields of the deflection coils. Since a thin walled metallictube may lack the necessary strength and rigidity, alternatively, acomposite tube having a fiber glass outer tube containing a layer ofnonmagnetic stainless steel or inner tube may be provided, the fiberglass serving to reinforce and strengthen the tube structure.

In addition, novel spray apertures may be provided intermediate thetube, to block the path of electrons reflected from the walls of thetube, and to thereby improve the beam spot size. Moreover, the sprayapertures tend to increase the vacuum impedance of the tube so as tofacilitate differential pumping of the specimen chamber and electron gunhousing.

It is thus an object of the present invention to provide a scanningelectron microscope employing a vacuum containing tube interior of theelectron-optical column, the tube being removably mounted to permitremoval upon disassembly of only one end of the electronoptical column.

Another object of the present invention is to provide a scanningelectron microscope employing a vacuum containing tube interior of theelectron-optical column, the tube comprising a thin walled nonmagneticstainless steel tube having a wall thickness of less than ten mils, inorder to minimize eddy current losses while eliminating electrostaticcharging.

Yet another object of the present invention is to provide a scanningelectron microscope employing a tube interior of the electron-opticalcolumn having novel spray apertures interior of the tube.

These and other objects, features and advantages of the presentinvention will be more readily apparent from the following detaileddescription of the present invention with reference to the accompanyingdrawings wherein:

FIG. 1 is a side, cross-sectional view of the electronoptical column ofa scanning electron microscope according to the present invention;

FIG. 2 is an enlarged side, crosssectional view of the upper tube flangeportion of the apparatus of FIG. 1;

FIG. 3 is an enlarged side, crosssectional view of the lower tube endportion of the apparatus of FIG. 1;

FIG. 4 is a side, cross-sectional view, similar to FIG. 3, of analternative embodiment of the present invention;

FIG. 5 is a side, cross-sectional view, similarto FIG. 3, of anotherembodiment of the present invention;

FIG. 6 is a side, cross-sectional view, similar to FIG. 3, of stillanother embodiment of the present invention;

FIG. 7 is a side, cross-sectional view of the spray aperture portion ofthe apparatus of FIG. 1; and

FIG. 8 is a side, cross-sectional view illustrating the insertion of thespray aperture of FIG. 7.

Referring initially to FIG. 1, there is shown the electron-opticalcolumn A of a scanning electron microscope according to the presentinvention. Electronoptical column A comprises an electron gun housing 10in which an electron source 11 is disposed. Electron gun housing 10comprises a substantially cylindrical housing closed at one end, andhaving an angular groove at the other end adapted to contain an O-ringor seal 12. Electron gun housing 10 includes a pumping port 13 which isemployed to evacuate the interior of the electron-optical column, aswill be described in greater detail hereinafter.

According to the preferred embodiment of the present invention, electrongun housing 10 is formed of upper and lower halves 10a and 10b,respectively, which are suitably mated by a clamp 14, a seal 15 beingdisposed therebetween.

Electron gun housing 10 is mounted by a clamp 16 to a first coil housingor pole piece 17, with O-ring 13 abutting housing 17. Housing 17comprises a hollow cylindrical housing having an interior wall 17adefining an annular chamber and an axial opening. Contained within theannular chamber is a coil 18. Inner wall 17a includes a pair ofnon-magnetic spacers or gaps 19 at the ends thereof and a non-magneticspacer or gap 20 approximately in the middle thereof. Coil 18, whenappropriately energized, cooperates with housing 17 to form threemagnetic lens fields at gaps 19 and 20. These magnetic lens fields serveto focus the beam of electrons emitted by electron source 11.

Disposed within the axial opening in housing 17, and extending therebeyond, is a tube 21. Tube 21 is attached at one end to a flange 22. Aswill be described in greater detail hereinafter, flange 22 mounts to theupper surface of housing 17 and functions to mount and vacuum seal tube21. An apertured cup 23 is mounted on flange 22, the beam of electronsemitted by electron source 11 being directed through the aperture in cup23 and thence through the bore of tube 21. Accordingly, the aperture ofcup 23 functions as the initial aperture of the electron-optical column,and thus functions, in part, to define the solid angle of the beam ofelectrons emitted by electron source 11.

Disposed around that portion of tube 21 which protrudes beyond housing17 are an astigmator coil 24 and a pair of deflection coils 25. As willbe described in greater detail hereinafter, deflection coils 25 aresuitably energized to cause the beam of electrons to scan the specimenin the desired manner.

A second housing 26, somewhat similar to housing 17, is disposed arounddeflection coils 25 and astigmator coil 24, and is mounted to housing 17by a clamp 27. In particular, housing 26 comprises a hollow cylindricalhousing having an inner wall 26a defining an annular chamber and anaxial opening. The axial opening within inner wall 26a substantiallyencloses astigmator coil 24 and deflection coils 25, tube 21 thuspassing therethrough. Contained within the annular chamber of housing 26is a coil 28. Inner wall 26a of housing 26 includes a non-magneticspacer or gap 29 between interior wall 26a and the lower end of wall ofhousing 26. When suitably energized, coil 28 cooperates with housing 26to form a magnetic lens field at gap 29, which magnetic lens field willfocus the beam of electrons in a manner to be described in greaterdetail hereinafter.

Attached to the other or lower end of tube 21 is a flange 30. Flange 30cooperates with the lower end wall of housing 26 to mount and seal tube21, in a manner to be described in greater detail hereinafter.

Disposed beneath housing 26 is a specimen chamber 31. Specimen chamber31 comprises a substantially cylindrical chamber closed at one end, and,at the other end having an annular groove containing an O-ring or seal32. Specimen chamber 31 is suitably disposed beneath housing 26 so thatO-ring 32 abutes the lower end of housing 26, thus forming a vacuum sealbetween housing 26 and specimen chamber 31. Specimen chamber 31 furtherincludes a pumping port 33, which is employed to evacuate the interiorof the electron-optical column.

Disposed within specimen chamber 31 is a stage 34 adapted to support aspecimen 35 in a predetermined position relative to the electron beam.An electron collector 36 is disposed adjacent specimen 35 and funcftions to receive the electrons emitted by, or reflected from, specimen35 as it is scanned by the electron beam. Electron collector 36functions in cooperation with electronic circuitry (not shown) toprovide a conventional scanning electron microscope display.

Referring now to FIGS. 2 and 3, tube 21 will now be described in greaterdetail. Specifically, tube 21 is mounted for vacuum-containing sealingat the ends thereof, so that the vacuum contained within theelectron-optical column A is confined within tube 21, electron gunhousing 10 and specimen chamber 31. Tube 21 may comprise either ametallic or non-metallic tube. However, if a non-metallic or insulatingtube is employed, electrostatic charges may develop in the interiorthereof, which charges may result in astigmatism, defocusing ormisdeflection of the electron beam. If, however, a metal tube 21 isemployed, the varying fields produced by deflection coils 25 necessaryto appropriately deflect the beam of electrons, will induce eddycurrents in tube 21, which currents will unduly limit the usefulscanning frequency and otherwise deleteriously affect the operation ofscanning electron microscope.

Accordingly, it is preferable that tube 21 comprise a thin-wallednon-magnetic metal tube. Specifically, applicants have found that tube21 may preferably comprise a non-magnetic stainless steel tube having awall thickness of less than mils, so as to eliminate electrostaticcharging, while minimizing eddy current losses. Applicants have foundthat such a tube 21 may successfully be employed with scanning rates inexcess of 15,000 lines per second. It is further apparent that evenhigher scanning rates may be achieved with a thinner tube 21.

As the wall thickness of tube 21 decreases, it is apparent that thestrength and rigidity of tube 21 decreases. Thus, according to analternative embodiment of the present invention, tube 21 may comprise acomposite tube formed by an outer insultaing tube encompassing a thinmetallic layer or inner tube. In this manner, the problem ofelectrostatic charging of the inside of tube 21 is eliminated, as theinner metallic tube may be grounded to conduct away the charges, whilethe eddy current problem is minimized, since the inner tube may beextremely thin. Thus, tube 21 may be a composite formed by a thin,non-magnetic stainless steel inner tube contained within an outerfiberglass tube, which serves to provide the necessary strength andrigidity.

Referring specifically to FIG. 2, the mounting and sealing of tube 21will now be described in greater detail. As briefly referred tohereinbefore, one end of tube 21 is attached to a flange 22 which ismounted and vacuum sealed against the upper surface of housing 17.Specifically, flange 22 includes a portion having a diameter greaterthan the aperture in the upper surface of housing 17. The upper surfaceof housing '17 includes an annular groove containing an O-ring or seal37. A pair of screws 38, threadably engaging the upper surface ofhousing 17, urge apertured cup 23 and flange 22 downwardly to securetube 21 in the electron-optical column A while urging flange 22 intovacuum containing pressure contact with O-ring 37. Accordingly, theupper end of tube 21 is thus vacuum sealed to the upper surface ofhousing 17 by screws 38 and seal 37. Furthermore, as will be morereadily apparent hereinafter, screws 38 cooperate with flange 22 to urgetube 21 downwardly, the downward urging functioning to seal tube 21 tothe other end thereof.

Referring specifically to FIG. 3, the mounting and sealing of the lowerend of tube 21 will now be described in greater detail. As brieflyreferred to hereinbefore, the lower end of tube 21 is attached to aflange 30 which is vacuum sealed and mounted to the lower end of housing26. According to the present invention, the mounting and sealing oflower end of tube 21 is accomplished in a self-sealing manner as toeliminate the need for manual manipulation of the lower end of the tube.In this manner, tube 21 may be removed for cleaning or replacement upondisassembly of only one end of the electron-optical column.Specifically, according to the preferred embodiment of the present invention, only the electron gun housing 10 need be removed from theelectron-optical column A in order to remove tube 21. Of course, it isto be expressly understood that the particular end of theelectron-optical column through which the tube may be removed can bereversed, and such modification is specifically within the scope of thepresent invention.

According to the preferred embodiment of the present invention, thenon-manipulative for self-sealing mounting of the lower end of tube 21is accomplished by employing the downward urging from the upper end oftube 21, created by screws 38, to urge insulating flange 30 into vacuumsealing engagement with the lower end of housing 26. Specifically,flange 30 includes an annular groove in the lower surface thereofadapted to contain an O-ring or seal 39. Downward urging of tube 21,caused by the downward pressure at the other end of the tube, urges thelower face of flange 30 into contact with housing 26, therebycompressing O-ring or seal 39, so as to achieve the desiredvacuumcontaining seal. Accordingly, it is apparent that by so providinga non-manipulative seal at one end of the tube 21, the tube may beremoved upon disassembly of only one end of the electron-optical columnA.

Flange 30 may additionally function to mount the final aperture for theelectron beam. Specifically, flange 30 may include a cylindrical recessin the lower surface thereof, and an apertured disc 40 may be mountedtherein. Specifically, a resilient C-ring 41 may be provided to retaindisc 40 in the recessing of flange 30. It is thus apparent that theaperture in disc 40 functions to further define the solid angle of thebeam of electrons passing therethrough. Of course, the final apertureneed not be provided within flange 30, but may be provided in accordancewith conventional scanning electron microscope construction.

Referring now to FIG. 4, an alternative nonmanipulative self-sealingarrangement for the lower end of tube 21 will now be described indetail. Specifically, according to this alternative embodiment, thelower end of housing 26 is provided with an annular groove containing aseal 42. The lower end of tube 21 is disposed on seal 42 and is urgedinto vacuum sealing contact therewith by the downward urging of tube 21from the top thereof. Specifically, as referred to hereinbefore, screws38 urge tube 21 downwardly, so that the downward pressure from the topend of tube 21 seals the lower end thereof. It is apparent thataccording to this embodiment of the present invention, no lower flangeneed be provided for tube 21, as the seal therefore may be located inhousing 26 rather than in a lower flange. Of course, a final aperture(not shown) may be provided in accordance with conventional scanningelectron microscope construction.

Referring now to FIG. 5, another alternative nonmanipulativeself-sealing arrangement for the lower end of tube 21 will now bedescribed in detail. Specifically, the lower'end of tube 21 may besealed in a manner which eliminates the need for manual manipulation byemploying radial pressure on the seal rather than downward pressure aspreviously described. To this end, the lower end of housing 26 mayinclude a cylindrical projection 43 extending upwardly into the interiorof housing 26. Cylindrical projection 43 has a diameter slightly smallerthan the inside diameter of tube 21.

Cylindrical projection 43 includes an annular groove in the outersurface thereof for containing an O-ring or seal 44. As is apparent fromFIG. 5, tube 21 is seated on cylindrical projection 43 with seal 44engaging the inside surface of tube 21 toward the end thereof. Seal 44has a nominal diameter greater than the inner diameter of tube 21, sothat the engagement of tube 21 over cylindrical projection 43 willresult in the radial compression of seal 44 thereby resulting invacuumcontaining sealing of the lower end of tube 21. Accordingly, it isapparent that a non-manipulative seal for the lower end of tube 21employing radial pressure, rather than axial pressure as previouslydescribed, may be provided. Once again, a final aperture (not shown) maybe provided in accordance with conventional scanning electron microscopeconstruction.

Referring now to FIG. 6, still another alternative embodiment of thenon-manipulative self-sealing arrangement of the lower end of tube 21will now be described in detail. Specifically, the sealing arrangementdepicted in FIG. 6 again employs radial pressure to seal the lower endof tube 21. To this end, the lower end of housing 26 is provided with acylindrical aperture having a diameter slightly greater than the outsidediameter of tube 21. An annular groove containing a seal 45 is providedon the inner surface of the cylindrical aperture. Seal 45 is resilientand has a nominal inside diameter slightly smaller than the outsidediameter of tube 21. Thus, the lower end of tube 21 is engaged in the cylindrical aperture in housing 26, as depicted in FIG. 6. Tube 21 willthus press outwardly upon seal 45, thereby urging seal 45 intovacuum-containing sealing with the outside of tube 21. Accordingly, itis apparent that a non-manipulative seal for the lower end of tube 21employing radial pressure on the sealing element may be provided. Onceagain, a final aperture (not shown) may be provided in accordance withconventional scanning electron microscope construction.

According to a further aspect of the present invention, spray aperturesadapted to block the path of electrons reflected off the interiorsurface of tube 21 may be provided within tube 21. Specifically,applicants have found it desirable to employ a spray aperture withintube 21 at a region slightly before the astigmator and deflection coils24 and 25, along the path of the electron beam. Applicants have found,however, that the placement of the spray apertures within tubes 21 isnot critical, with the exception that the spray apertures should not belocated in the region of the deflection coils, since the deflection ofthe beam might thus be interfered with.

Referring specifically to FIGS. 7 and 8, the spray apertures accordingto the present invention will now be described in detail. Specifically,according to the present invention, spray apertures are provided in theform of a helical metallic ribbon 46 engaging the interior of tube 21,as depicted in FIG. 7. The helical ribbon 46 presents an inner apertureand an outer metal surface to the electron beam, so that electrons whichhave diverged from the desired path of the electron beam, includingthose electrons reflected off the interior surface of tube 21, willimpinge upon the metallic surface of helical ribbon 46, while thoseelectrons following the desired beam path will pass through the centralaperture unimpeded. As will be more readily apparent hereinafter, use ofa helical ribbon 46 as a spray aperture is advantageous in that thehelical ribbon 46 may readily be inserted into the tube 21 and will beretained therein due to the resilient urging of the helical ribbon 46against the inside wall of tube 21. Thus, no independent mounting needbe provided for spray apertures in accordance with the presentinvention. Moreover, the spray aperture 46 in the interior of tube 21introduces a substantial vacuum impedence into tube 21, so thatelectron-optical column may readily be differentially pumped inaccordance with conventional scanning electron microscope techniques.

Referring specifically to FIG. 8, the insertion of the helical ribbon 46into the tube 21 will now be described in greater detail. Helical ribbon46 has a nominal outer diameter slightly greater than the insidediameter of tube 21. In order to insert helical ribbon 46 into tube 21,it is necessary to radially compress the ribbon 46. To this end, a rod47 having a diameter substantially equal to the bore of helical ribbon46 may be provided, and ribbon 46 may be disposed about rod 47. Helicalspring 46 may then be longitudinally extended which will result in theradial compression thereof. Rod 47 may then be introduced into tube 21through one of the ends thereof, so as to locate ribbon 46 at itsdesired location. A second rod 48 having a bend or hook on the endthereof may then be introduced into the tube 21 through the other endthereof and the hook or bend engaged with ribbon 46, as depicted in FIG.8, while maintaining ribbon 46 in position through the use of rod 48,rod 47 may then be drawn outwardly, leaving helical ribbon in positionwithin tube 21. Rods 47 and 48 may then be manually manipulated tolongitudinally compress helical ribbon 46, thereby resulting in theradial expansion thereof. The radial expansion functions to urge theperiphery of helical ribbon 46 into resilient engagement with the innersurface of tube 21, and to thereby result in the desired mountingthereof, as depicted in FIG. 7. Accordingly, it is apparent that thespray apertures according to the present invention do not requirecomplex mounting procedures, such as welding, soldering or the like,which would be relatively unfeasible within the interior of tube 21.

In operation, a specimen 35 is introduced into the interior of specimenchamber 31 through a hatch or air lock (not shown), and is suitablyattached to stage 34. Vacuum pumps (not shown) are attached to pumpingports 13 and 33, so as to substantially evacuate the interior ofspecimen chamber 31, tube 21 and electron gun housing 10. A high voltagesource of electricity is connected to electron source 11, so as to causean electron beam to be emitted therefrom. Suitable DC voltages areapplied to magnet coils l8 and 28 so as to form magnetic lens fields atgaps 19, 20 and 29, in a conventional manner. Of course, housing 17 and26 have heretofore been adjusted and are aligned with respect to tube21, so that the magnetic lens fields will function to focus the beam ofelectrons emitted from electron As referred hereinbefore, tube 21 may begrounded so as to conduct away electrostatic charges developed thereon.Moreover, the spray aperture of helical ribbon 46 provided within tube21 will function to collect any stray electrons diverging from the pathof the electron beam, such as those which may be reflected off the innersurface of tube 21. Thereby resulting in a relatively precise electronbeam. If, during the course of operations, materials are deposited onthe interior of tube 21 due to vacuum deposition or evaporation, thetube 21 may be removed for cleaning or replacement by disassembling onlythe electron gun housing 10 to provide access to screws 38 and flange22. It is noteworthy that the specimen chamber need not be disassembled,as the lower seal of tube 21 does not require manual manipulation fordisassembly. Thus, tube 21 may be withdrawn from the electron-opticalcolumn A from the specimen chamber thereof. Thereafter, tube 21 may becleaned or replaced, with minimal interference with the alignment of themagnetic lenses, and with minimal possibilities of causing damagethereto.

In addition, the foregoing construction of an electron-optical columnfor a scanning electron microscope is advantageous in that the volume tobe evacuated is reduced, and the number of seals are minimized, therebyreducing the pump-down time and minimizing the possibilities of leakage.Furthermore, since the magnetic pole pieces need no longer be machinedfor vacuum sealing, a substantial manufacturing cost reduction isachieved.

While particular embodiments of the present invention have been shownand described, it is to be understood that modifications or adaptationsmay be made without departing from the true spirit and scope of theinvention, as set forth in the claims.

What is claimed is:

1. A scanning electron microscope comprising an electron source housing,electron source means for emitting a beam of electrons disposed interiorof said electron source housing, an electron-optical column including aplurality of magnetic coils having a bore thercthrough, said electronsource housing being removably mounted to one end of saidelectron-optical column, a chamber for containing a specimen, saidchamber being mounted to the other end of said electron-optical column,detector means for detecting electrons emitted or reflected by saidspecimen, a unitary tube removably disposed interior of saidelectronoptical column interconnecting said electron source housing andsaid chamber, said beam of electrons being directed through said tube tosaid specimen, mounting means for removable mounting and sealing saidtube at one end thereof, self-sealing means for nonmanipulativelyself-sealing said tube at the other end thereof, said other end of saidtube being dimensioned smaller than said bore to permit removal of saidtube through said bore without manual manipulation or disassembly atsaid other end, means for evacuating the interior of said electronsource housing and of said tube and of said chamber, and means forenergizing said coils for focus and'scan said beam of electrons on saidspecimen.

2. Apparatus according to claim 1 wherein said selfsealing meanscomprises a resilient seal longitudinally disposed beyond said other endof said tube and wherein said mounting means comprises meanslongitudinally urging said tube toward said other end for vacuum sealingengagement of said other end and said seal.

3. Apparatus according to claim 2 comprising a flange disposed on saidother end of said tube, said flange having a diameter smaller than thebore of said electron-optical column, said flange including an annulargroove containing said seal.

4. Apparatus according to claim 1 wherein said selfsealing meanscomprises a resilient seal radially disposed around said other end ofsaid tube, said seal having a nominal inner diameter smaller than theouter diameter of said tube.

5. Apparatus according to claim 1 wherein said selfsealing meanscomprises a resilient seal radially disposed interior of the other endof said tube, said seal having a nominal outer diameter greater than theinner diameter of said tube.

6. Apparatus according to claim 1 wherein said one end of said tube isadjacent said electron source housing.

7. Apparatus according to claim 1 comprising a helical metal ribbonhaving a nominal outer diameter greater than the inner diameter of saidtube, disposed in said tube to form a spray aperture.

8. Apparatus according to claim 1 comprising a spray aperture disposedinterior of said tube.

9. Apparatus according to claim 8 wherein said spray aperture comprisesmounting means resiliently radially urging against-the interior of saidtube.

10. In a scanning electron microscope having an electron source housing,electron source means for emitting a beam of electrons disposed interiorof said electron source housing, an electron-optical column including aplurality of magnetic coils, said electron source housing beingremovably mounted to one end of said electron-optical column, a chamberfor containing a specimen, said chamber being mounted to the other endof said electron-optical column, a unitary tube disposed interior saidelectron-optical column, said beam of electrons being directed throughsaid tube to said specimen, means for evacuating the interior of saidelectron source housing and of said tube and said chamber, detectormeans for detecting electrons emitted or reflected by said specimen, theimprovement comprising: a spray aperture disposed interior of said tubehaving mounting means resiliently radially urging against the interiorof said tube.

11. Apparatus according to claim 10 wherein said spray aperturecomprises a helical'metal ribbon having a nominal outer diameter greaterthan the inner diameter of said tube.

12. In a scanning electron microscope having an electron source housing,electron source means for emitting a beam of electrons disposed interiorof said electron source housing, an electron-optical column including aplurality of magnetic coils having a bore therethrough, said electronsource housing being removably mounted to one end of saidelectron-optical column, a chamber for containing a specimen, saidchamber being mounted to the other end of said electron-optical column,detector means for detecting electrons emitted or reflected by saidspecimen, a unitary tube disposed interior of said electron-opticalcolumn, said beam of electrons being directed through said tube to saidspecimen, means for evacuating the interior of said electron sourcehousing and of said tube and of said chamber, the improvementcomprising: mounting means for removably mounting and sealing said tubeat one end thereof and self-sealing means for sealing said tube at theother end thereof, said other end of said tube being dimensioned smallerthan said bore to permit removal of said tube through said bore withoutdisassembly or manual manipulation at said other end.

13. Apparatus according to claim 12 wherein said self-sealing meanscomprises a resilient seal longitudinally disposed beyond said other endof said tube and wherein said mounting means comprises meanslongitudinally urging said tube toward said other end for vacuum sealingengagement of said other end and said disposed interior of the other endof said tube, said seal having a nominal outer diameter greater than theinner diameter of said tube.

1. A scanning electron microscope comprising an electron source housing,electron source means for emitting a beam of electrons disposed interiorof said electron source housing, an electronoptical column including aplurality of magnetic coils having a bore therethrough, said electronsource housing being removably mounted to one end of saidelectron-optical column, a chamber for containing a specimen, saidchamber being mounted to the other end of said electron-optical column,detector means for detecting electrons emitted or reflected by saidspecimen, a unitary tube removably disposed interior of saidelectron-optical column interconnecting said electron source housing andsaid chamber, said beam of electrons bEing directed through said tube tosaid specimen, mounting means for removable mounting and sealing saidtube at one end thereof, self-sealing means for nonmanipulativelyself-sealing said tube at the other end thereof, said other end of saidtube being dimensioned smaller than said bore to permit removal of saidtube through said bore without manual manipulation or disassembly atsaid other end, means for evacuating the interior of said electronsource housing and of said tube and of said chamber, and means forenergizing said coils for focus and scan said beam of electrons on saidspecimen.
 2. Apparatus according to claim 1 wherein said self-sealingmeans comprises a resilient seal longitudinally disposed beyond saidother end of said tube and wherein said mounting means comprises meanslongitudinally urging said tube toward said other end for vacuum sealingengagement of said other end and said seal.
 3. Apparatus according toclaim 2 comprising a flange disposed on said other end of said tube,said flange having a diameter smaller than the bore of saidelectron-optical column, said flange including an annular groovecontaining said seal.
 4. Apparatus according to claim 1 wherein saidself-sealing means comprises a resilient seal radially disposed aroundsaid other end of said tube, said seal having a nominal inner diametersmaller than the outer diameter of said tube.
 5. Apparatus according toclaim 1 wherein said self-sealing means comprises a resilient sealradially disposed interior of the other end of said tube, said sealhaving a nominal outer diameter greater than the inner diameter of saidtube.
 6. Apparatus according to claim 1 wherein said one end of saidtube is adjacent said electron source housing.
 7. Apparatus according toclaim 1 comprising a helical metal ribbon having a nominal outerdiameter greater than the inner diameter of said tube, disposed in saidtube to form a spray aperture.
 8. Apparatus according to claim 1comprising a spray aperture disposed interior of said tube.
 9. Apparatusaccording to claim 8 wherein said spray aperture comprises mountingmeans resiliently radially urging against the interior of said tube. 10.In a scanning electron microscope having an electron source housing,electron source means for emitting a beam of electrons disposed interiorof said electron source housing, an electron-optical column including aplurality of magnetic coils, said electron source housing beingremovably mounted to one end of said electron-optical column, a chamberfor containing a specimen, said chamber being mounted to the other endof said electron-optical column, a unitary tube disposed interior saidelectron-optical column, said beam of electrons being directed throughsaid tube to said specimen, means for evacuating the interior of saidelectron source housing and of said tube and said chamber, detectormeans for detecting electrons emitted or reflected by said specimen, theimprovement comprising: a spray aperture disposed interior of said tubehaving mounting means resiliently radially urging against the interiorof said tube.
 11. Apparatus according to claim 10 wherein said sprayaperture comprises a helical metal ribbon having a nominal outerdiameter greater than the inner diameter of said tube.
 12. In a scanningelectron microscope having an electron source housing, electron sourcemeans for emitting a beam of electrons disposed interior of saidelectron source housing, an electron-optical column including aplurality of magnetic coils having a bore therethrough, said electronsource housing being removably mounted to one end of saidelectron-optical column, a chamber for containing a specimen, saidchamber being mounted to the other end of said electron-optical column,detector means for detecting electrons emitted or reflected by saidspecimen, a unitary tube disposed interior of said electron-opticalcolumn, said beam of electrons being directed through said tube to saidspecimen, means for evacuating The interior of said electron sourcehousing and of said tube and of said chamber, the improvementcomprising: mounting means for removably mounting and sealing said tubeat one end thereof and self-sealing means for sealing said tube at theother end thereof, said other end of said tube being dimensioned smallerthan said bore to permit removal of said tube through said bore withoutdisassembly or manual manipulation at said other end.
 13. Apparatusaccording to claim 12 wherein said self-sealing means comprises aresilient seal longitudinally disposed beyond said other end of saidtube and wherein said mounting means comprises means longitudinallyurging said tube toward said other end for vacuum sealing engagement ofsaid other end and said seal.
 14. Apparatus according to claim 12wherein said self-sealing means comprises a resilient seal radiallydisposed around said other end of said tube, said seal having a nominalinner diameter smaller than the outer diameter of said tube. 15.Apparatus according to claim 12 wherein said self-sealing meanscomprises a resilient seal radially disposed interior of the other endof said tube, said seal having a nominal outer diameter greater than theinner diameter of said tube.